GE CGA 351, User Manual

Page: 112 / 133

December 2004
A-2 The Nernst Equation
The Model CGA 351
Nernst Equation
The Nernst Equation specifically applicable to the CGA 351 analyzer
is obtained by substituting the above values into the general equation,
converting the natural logarithm to the common logarithm (base 10)
and converting the units for E
12
to millivolts. This results in the
following equation:
(A-2)
The CGA 351 measures the temperature of the oxygen sensor and
automatically inserts the correct value into the Nernst Equation. For
example, at the standard oxygen sensor operating temperature of
700 °C, Equation A-2 above becomes:
(A-3)
The voltage drop across the zirconium oxide sensor, as calculated
from the Nernst equation, is then sent to the linearizer circuit. The
circuit produces a linear analog output that represents the percentage
of oxygen in the sample gas, and this signal is available for display on
the digital panel meter and/or for output to an external device.
For convenience, the standard Nernst equation for the CGA 351
zirconia oxygen analyzer (Equation A-3 above) has been converted
into a graphical format. Use this graph (see Figure A-1 on page A-3)
to quickly correlate the analog output voltage reading with the
corresponding oxygen percentage, at the standard 700°C operating
temperature of the meter. In addition, the data is presented in tabular
form in Table A-1 on page A-4. This table is useful for looking up the
correct mV value to enter during the calibration procedure, if the data
will not be entered directly in either ppm or pcnt oxygen.
E
12
mV ( ) 0.049605 T
•
0.2093
p
2
----------------
log
•
=
E
12
mV ( ) 48.273
0.2093
p
2
----------------
log
•
=
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